Shaped glass sheets are often used as solar concentrating mirrors. Back-silvered shaped glass mirrors are preferred for this use, as they are proven to maintain high reflectivity in field operation, over many years of exposure. They are also mechanically stable, holding their shape also over many years, with little yielding or fatigue.
In the past, nearly all curved solar concentrating mirrors have been shaped by rollers after heating and softening. However, this method may be limited in its shape accuracy, typically to >1.5 mrad rms slope error, and may be limited to shapes that are cylindrical or nearly cylindrical, i.e. curved mostly in one dimension and used to concentrate sunlight to a line focus.
In the past, most curved glass solar concentrating mirrors forming a point focus were made by sagging into a rigid, dish-shaped, steel mold, for example, in U.S. Pat. No. 8,082,755, to Angel and Olbert. High accuracy has been demonstrated by this method (<1 mrad rms slope error). However, high accuracy shaping may sometimes involve a mold made by machining and refiguring of the machined mold surface to improve fine scale surface accuracy, which can be time-consuming in some applications. If the replica is formed when neither the sheet or panel, nor the mold is in thermal equilibrium, then the replica shape may deviate from that of the mold. The mold shape may need to be altered to improve larger scale surface errors. Furthermore, large rigid molds may need to be thick to hold their shape accuracy, and as a result, can have high thermal inertia, which makes it difficult to rapidly heat a mold having high thermal inertia without exacerbating the distortion from thermal gradients. As a result, processing cycle times may be slowed.
In the past, the industry has used solar reflectors curved to very few different shapes, because the manufacturing methods require retooling for different shapes, an expensive and time consuming process. This has inhibited development of new concentrator applications which require unusual shapes, or those that benefit from use of a mosaic of many different replica shapes.
In addition, shaped metal panels are often used to make large diameter microwave antennae. Typically such an antenna dish will be fabricated from panel segments of many different shapes. Such panels may be difficult and expensive to manufacture, each differently shaped panel being replicated from a mold of different shape. Furthermore, a panel in an antenna operated in the shorter wavelengths of the microwave range, around 1 cm or less, may require backing with a stiffened structure to hold its shape to the required accuracy. In the past, each panel of different shape has required a differently shaped backing structure, further driving up the cost of manufacture.